Removable shroud and pump deck for a boiling water nuclear reactor

ABSTRACT

In a pressure vessel of a nuclear reactor containing a core assembly enclosed within a core shroud, the core shroud spaced radially inwardly of a side wall of the pressure vessel with an annular pump deck located in an annular radial space between the core shroud and the side wall of the pressure vessel, the improvement wherein the shroud is removably secured to an annular support leg extending upwardly from the bottom of the pressure vessel; and further wherein the annular pump deck is provided in the form of a plurality of removable segments.

TECHNICAL FIELD

This invention relates to boiling water nuclear reactors andspecifically, to a new core shroud and pump deck design which allows foreasy removal and/or replacement of these reactor structural componentswhen damaged or otherwise in need of repair or replacement.

BACKGROUND

Typical boiling water nuclear reactors include a reactor assembly whichconsists of the reactor vessel and its internal components including thecore, core shroud, top guide assembly, core plate assembly, steamseparator and dryer assemblies, and jet pumps. Also included in thereactor assembly are the control rods, control rod drive housings andthe control rod drives.

The reactor vessel is a generally cylindrical pressure vessel (RPV) witha single full diameter removable head. The shroud is a cylindricalstainless steel structure located within the RPV and which surrounds thecore, providing a barrier to separate the upward flow through the corefrom the downward flow in the annulus between the RPV wall and the coreshroud. The conventional core shroud is welded to the bottom of the RPVand supports the weight of the top guide, core plate and shroud headalong with attached steam separators.

Recent discoveries of unexpected circumferential cracks propagatingthrough the thickness of the shrouds in relatively young operating BWR'shas prompted a re-design of the core shrouds for future BWR's.

The primary cause of the observed cracking has been intergranular stresscorrosion in the heat affected zones near many of the horizontal weldsof the shroud and shroud supports. There have also been some cracksobserved in the mid-belt regions of BWR shrouds, and these have beenthought to be caused by irradiation assisted stress corrosion.

The current advanced boiling water reactor (ABWR) shroud, like theconventional BWR shroud, is permanently welded to the bottom of thevessel and is not intended to be removed or replaced. The shroud in theABWR has various horizontal welds similar to the BWR shrouds in currentoperating plants, and therefore may also be at risk of similar stresscorrosion cracking problems.

The conventional pump deck section is permanently welded in placebetween the shroud and the reactor pressure vessel, and it too is notintended to be removed or replaced. As with the shroud welds, the pumpdeck welds are also susceptible to stress corrosion cracking.

SUMMARY OF THE INVENTION

This invention relates to a new and improved shroud and pump deck designwhich allows for the relatively easy removal of both components. Inaccordance with the invention, both the shroud and pump deck are boltedto the top of the shroud support leg extending upwardly from the bottomof the RPV. Specifically, the shroud has a radially inwardly extendingflange at its bottom end for bolting into the upper cylindrical sectionof the shroud support leg. The bolts restrain vertical loading on theshroud while horizontal loading is restrained by wedges mounted betweenthe shroud and a plurality of wedge support blocks on the pump deck.Thus, the inner diameter portion of the pump deck is verticallysandwiched between the shroud flange and the upper edge of thecylindrical section of the shroud support leg. At the same time, theouter diameter of the pump deck is held in place within a radiallyinwardly facing groove on the inside wall of the reactor pressurevessel. The pump deck is formed in ten separate segments with a reactorinternal pump diffuser adapted to extend vertically through an openingin the middle of each segment.

In a further feature of the invention, a keyed pump deck segment locksthe remaining nine deck segments of the pump deck in place. It is thelast portion of the pump deck to be installed and the first portion ofthe pump deck to be removed. This keyed segment is very similar inconstruction to the other nine segments; however, it is bolted into asupport ledge on the reactor pressure vessel wall as opposed simply tobeing held by the groove on the reactor pressure vessel wall as in thecase of the other nine pump deck segments.

From the above, it will be appreciated that after the shroud bolts havebeen removed, the shroud may be lifted vertically from the core. Thekeyed pump deck segment can then be removed, followed by the remainingpump deck segments.

Thus, in accordance with one aspect of the subject invention, there isprovided in a pressure vessel of a nuclear reactor containing a coreassembly enclosed within a core shroud, the core shroud spaced radiallyinwardly of a side wall of the pressure vessel with an annular pump decklocated between the core shroud and the side wall of the pressurevessel, the improvement wherein the shroud is removably secured to anannular support of the pressure vessel.

In accordance with another aspect, the subject invention relates to apressure vessel of a nuclear reactor containing a core assembly enclosedwithin a core shroud, the core shroud spaced radially inwardly of a sidewall of the pressure vessel with an annular pump deck located betweenthe core shroud and the side wall of the pressure vessel, theimprovement wherein the annular pump deck is provided in the form of aplurality of removable segments.

In accordance with still another aspect, the invention relates to apressure vessel of a nuclear reactor containing a core assembly enclosedwithin a core shroud, the core shroud spaced radially inwardly of a sidewall of the pressure vessel with an annular pump deck located in anannular radial space between the core shroud and the side wall of thepressure vessel, the improvement wherein the shroud is removably securedto an annular support leg extending upwardly from the bottom of thepressure vessel; and further wherein the annular pump deck is providedin the form of a plurality of removable segments.

It will thus be appreciated that the invention provides a shroud and apump deck capable of being easily removed and reused or replaced. Theinvention also provides a wedge mechanism to transfer the horizontalloads of a core shroud to the pump deck or other supporting member. Theconstruction in accordance with this invention also results in mosthorizontal welds being located in otherwise removable elements,facilitating repair and/or replacement of faulty welds.

Other objects and advantages of the subject invention will becomeapparent from the detailed description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation, partly in section, illustrating aconventional boiling water reactor including a welded-in type coreshroud;

FIG. 2 is an enlarged detail taken from the lower left corner of thereactor shown in FIG. 1;

FIG. 3 is an enlarged detail similar to that shown in FIG. 2, and takenalong the section line 3--3 Of FIG. 5, and illustrating a removableshroud and pump deck in accordance with an exemplary embodiment of thisinvention;

FIG. 4 is an enlarged section detail taken along the line 4--4 of FIG.5;

FIG. 5 is a partial section taken in plan of the removable shroud andremovable pump deck segments in accordance with the invention;

FIG. 6 is an enlarged sectional detail taken along the line 6--6 of FIG.8, illustrating a keyed segment of the pump deck;

FIG. 7 is an enlarged sectional detail taken along the line 7--7 of FIG.8, illustrating the wedge lock on the keyed segment of the pump;

FIG. 8 is a partial section, also taken in plan, similar to FIG. 6 butillustrating the keyed pump deck segment in accordance with thisinvention; and

FIG. 9 is a partial sectional detail similar to FIG. 4 but illustratingthe manner in which the shroud and pump deck are removed from thereactor pressure vessel.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to FIG. 1, an advanced boiling water reactor (ABWR) 10 isillustrated which incorporates a welded-in type core shroud. Prior todiscussing the shroud configuration, however, it may be helpful tobriefly describe the reactor construction in general.

The reactor pressure vessel (or RPV) 12 is a substantially cylindricalvessel with a single full diameter removable head 14 bolted to thevessel as shown at 16. The RPV 12 houses the core shroud 18, the topguide assembly 20, the core plate assembly 22, steam separators 24 andsteam dryers 26. Also included in the reactor assembly are the reactorinternal pumps 28 (also referred to herein as jet pumps), control rods30 and associated drives 32. The control rods 30 occupy alternate spacesbetween fuel assemblies within the core in a conventional fashion, andmay be withdrawn into the guide tubes below the core during plantoperation.

An annular pump deck 34 extends around the core shroud 18, in theannular space or annulus 36 between the shroud 18 and the inside of theRPV sidewall. Pumps 28 project through the bottom of the RPV and includediffusers 38 which extend upwardly through pump openings or inlets 39 inthe deck 34 and into the annulus 36 as best seen in FIG. 2.

All major internal components of the conventional BWR type reactor canbe removed except the jet pump diffusers 38, the core shroud 18, the jetpumps 28 and the high pressure coolant injection inlet piping. Theinvention here specifically has to do with the core shroud 18 and thepump deck 34, best seen in FIG. 2. As shown there, the jet pumpdischarge diffusers (one shown at 38) penetrates the pump deck 34 belowthe core elevation to introduce the coolant into the inlet plenum orannulus 36. The pump deck 34 itself is welded to the vessel wall at 12'as well as to the thickened base portion 18A of the shroud 18 at 40. Theshroud base 38, in turn, is welded to an annular support leg 42 weldedto the bottom of the vessel at 44. Thus, it will be appreciated that theshroud 18 and the pump deck 34 are intended to be permanentinstallations in the conventional BWR construction.

With reference now to FIG. 3, a new removable core shroud 118 and pumpdeck 134 in accordance with this invention are illustrated. Forconvenience, similar reference numerals as used in FIGS. 1 and 2 areused in FIG. 3 where appropriate to identify corresponding elements, butwith the prefix "1" added. In accordance with this invention, theannular shroud support leg 142 now extends upwardly above the pump inlet139 and includes horizontal flow openings (one shown) 152. The supportleg 142 has an upper edge or support surface 154 which is adapted tosupport the pump deck 134. At the same time, the outer periphery of thepump deck 134 is supported within an interior, radially inwardly facingannular groove 156 in the RPV wall at 112', the base of which is at thesame height as surface 154 on support leg 142. This allows the pump deck134 to be supported horizontally within the annulus 136 via insertion inthe groove 156 and atop the support leg 142. It should be noted herethat the annular pump deck 134 is provided in the form of ten partannular segments, the annular extent of each of which is made apparentfrom FIGS. 5 and 8, as discussed further hereinbelow.

The shroud 118 and its thickened (but now axially shortened) base 118Aare provided with a radially inwardly directed annular flange ring 158which is welded to the bottom of the shroud base, and sized and locatedto seat fully on the surface 154. The flange ring 158 is formed with athrough bore 160 which is aligned with a bore 162 in the pump deck and abore 164 in the support leg 142 which opens into a radially inwardlyfacing, open recess 166. This arrangement allows a bolt 168 to passthrough the bore 160 of flange 158, through the bore 162 in the pumpdeck 134, and into the recess 166 where it is threadably secured by ablock nut 170 which is snugly received in the similarly shaped recess166. It will be appreciated that a plurality of such bolt holes andbolts are circumferentially spaced about the flange ring, as best seenin FIG. 6. These bolts 168 restrain vertical loading on the shroud 118.Each block nut 170 is provided with a bail 171 which facilitates removalof the block nut 170 remotely (from above) with the aid of a specializedlifting tool (not shown), after the bolt has been disengaged from thenut. Bolts 168 are also provided with hex heads 172, and locking hexheads 174 which may be spot welded upon assembly. By this arrangement,it is not necessary to tap directly into the support leg 142, and noadditional welds are required.

With specific reference now to FIGS. 4 and 5, a plurality of wedgesupport blocks 176 are located on the segments of the pump deck 134 (forexample, two wedge segment blocks per segment) at circumferentiallyspaced locations about the base portion 118A of the core shroud 118.These blocks 176 are radially outwardly spaced from the shroud base 118Aand have radially inner tapered surfaces 178 (best seen in FIG. 4). Thisallows space for wedge elements 180 (two per support block) to beinserted between the shroud and the support blocks. The wedges 180 eachhave a mating tapered surface 182 for engagement with surface 178 of therespective block 176. The base 118A of the core shroud 118 as well aseach block 176 are formed with aligned recessed grooves or keys 184,186, respectively, for receiving the wedge elements 180, as furtherdescribed below, and as best seen in FIG. 5. Each wedge element 180 isalso provided with a lifting bail 188 to facilitate removal of the wedgeelement, again, by remote tooling. It will be understood that thewedging action between blocks 176 and wedge elements 180 serves totransfer horizontal loading on the shroud to the pump deck.

As noted above, the annular pump deck 134 is divided into ten segments,nine of which are similar to that shown at 134A in FIG. 5. An otherwiseconventional KIP diffuser 138 is located in the middle(circumferentially) of each segment. Turning now to FIGS. 6, 7 and 8,the tenth pump deck segment 134B serves as a keylock segment, which isthe last segment installed and the first segment removed. This keyedsegment 134B does not seat in a groove in the RPV wall, but rather, isseated on an arcuate support ledge 190 formed by the RPV wall byremoving the upper part of the groove 156 in an arcuate portionsubstantially equal to the arcuate length of the segment 134B (FIG. 6).At annularly spaced locations along this keyed segment, the latter isbolted into the pressure vessel support ledge 190 as shown in FIGS. 7and 8. A bolt and block nut/bail assembly 192, similar to the bolt 168,block nut 170 assembly, is utilized to secure the keyed segment to theRPV support ledge 190. Note also the recess 194 in the RPV wall whichreceives the block nut of the assembly 192. Once again the block nut ofassembly 192 includes a bail element 196 to facilitate removal.

With reference now to FIG. 9, it will be appreciated that removal of allbolts 168 and wedge elements 180 will enable the core shroud 118 to belifted upwardly away from the core. The pump deck key segment 134B canthen be unbolted and removed. The remaining pump deck segments 134A canthen be pulled laterally out of the groove 156 in the RPV wall portion112' (after its corresponding RIP diffuser 138 has been removed).

It will be appreciated that, with the exception of the welds at thejuncture of the shroud support leg 142 and the bottom of the RPV,virtually all horizontal welds are contained in removable elements,i.e., the shroud 118, shroud base 138 and pump deck 134, so that faultywelds can be relatively easily repaired and/or replaced.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. In a pressure vessel of a nuclear reactorcontaining a core assembly enclosed within a core shroud, the coreshroud spaced radially inwardly of a side wall of the pressure vesselwith an annular pump deck located between the core shroud and the sidewall of the pressure vessel, the improvement wherein the annular pumpdeck is provided in the form of a plurality of removable segments. 2.The improvement of claim 1 wherein all but one of said segments areseated in a radially inwardly facing groove formed in said side wall ofthe pressure vessel.
 3. The improvement of claim 2 wherein said one ofsaid segments is removably fastened to a radially inwardly extendingshoulder of said side wall of the pressure vessel.
 4. The improvement ofclaim 1 wherein means are provided for transferring horizontal loads onthe core shroud to said pump deck.
 5. The improvement of claim 4 whereinsaid means comprises a plurality of wedge supports fixed to said pumpdeck and at least a corresponding plurality of wedge elementsfunctionally engaged between said wedge supports and said core shroud.6. The improvement of claim 5 wherein two wedge elements are providedfor each wedge support.
 7. In a pressure vessel of a nuclear reactorcontaining a core assembly enclosed within a core shroud, the coreshroud spaced radially inwardly of a side wall of the pressure vesselwith an annular pump deck located in an annular radial space between thecore shroud and the side wall of the pressure vessel, the improvementwherein the shroud is removably secured to an annular support legextending upwardly from the bottom of the pressure vessel; and furtherwherein the annular pump deck is provided in the form of a plurality ofremovable segments.
 8. The improvement of claim 7 wherein a radiallyinwardly directed flange ring is provided on the lower end of theshroud, said flange ring provided with a plurality of annularly spacedholes for receiving a corresponding number of fasteners used to securethe shroud to said annular support leg.
 9. The arrangement of claim 8wherein said pump deck is sandwiched between said flange ring and saidannular support leg at its radially inner end, and is supported within aradially inwardly facing groove about most of its radially outer end.10. The improvement of claim 7 wherein means are provided fortransferring horizontal loads on the core shroud to said pump deck. 11.The improvement of claim 10 wherein said means comprises a plurality ofwedge supports fixed to said pump deck and at least a correspondingplurality of wedge elements functionally engaged between said wedgesupports and said core shroud.
 12. The improvement of claim 11 whereintwo wedge elements are provided for each wedge support.